Computer storage systems often have dual, independent storage controllers for redundancy along with a number of disk drives. Dual, independent storage controllers are required to provide full data path redundancy to host computer systems. The controllers share access to the disk drives via their redundancy to host computer systems. The controllers share access to the disk drives via their respective drive ports. The respective host port connections may be connected to the same or different host systems. The controllers present the data on the drives to one or more host systems as one or more logical drive volumes.
Two major problems arise with using dual, independent storage controllers: coherency and latency. Coherency implies that the data in two or more memory locations is consistent and equivalent. Coherency problems arise since each controller has an independent memory system for caching data from the volumes. Latency is the time required to access data not including the actual transfer time. Data access latency problems arise since the controllers must make their respective caches coherent when the two controllers interleave access to the data in the volumes.
There are broadly three main solutions that have been devised to overcome these problems.
The first solution is to disallow multiple hosts from simultaneously accessing data. Memory requests from the hosts are queued or arbitrated by a processor to ensure only one host's access to memory at a given time. This solution restricts simultaneous data access by hosts connected to a single controller.
The second solution involves the two storage controllers sharing a common data cache between the controllers. Data may be simultaneously shared by the two storage controllers. A disadvantage of this approach is that the shared data cache is a single point of failure.
The third solution implements an ownership model which assigns to multiple controllers the ability to trade off data access privileges regarding data. Ownership entails permission to retrieve and alter data. A storage controller must initiate a request to obtain ownership of data before it can process the data. Ownership of data must be given when ownership of the data is requested by the storage controller. Only one controller may own a data block at any given moment. A disadvantage to this approach is that there are latencies associated with the ownership transfer. These latencies are visible to the host computer systems.
Because of the great increases in I/O requirements, present day bus-based I/O architectures such as PCI or PCI-X are becoming inadequate. Infiniband is an I/O specification to provide a channel based, switched fabric technology for computer systems that provides a scalable performance range of 500 MB/s to 6 GB/s per link to optimize I/O communications and provide adequate interfacing between known standards such as PCI and PCI-X. The specification is designed to provide scalability to satisfy the increasing demands of computer groups and flexibility to provide connectivity for various sized businesses independent of operating systems and computer and inter-operational flexibility. InfiniBand devices are compatible with a broad range of existing hardware devices and software operating systems for easier upgrading. InfiniBand is designed to connect servers with remote storage and networking devices as well as other servers. It is also designed for internal communication within servers.
Therefore, it would be desirable to provide a data storage controller architecture which offers coherency and avoids data access latency problems associated with simultaneous or interleaved data access to data on a given volume from two controllers. It would be desirable to utilize a high-speed switching mechanism, such as Infiniband, to achieve these goals.